Dielectric voltage divider



G. E. PlHL DIELECTRIC VOLTAGE DIVIDER 2 Sheets-Sheet 1 Aug. 12, 1958Filed March 4, 1954 Q emg e:

L 1 z E United States Patent DIELECTRIC VOLTAGE DIVIDER George E. Pihl,Abington, Mass., assignor, by mesne assignments, to Acton Laboratories,Inc., Acton, Mass.

Application March 4, 1954, Serial No. 414,206

1 Claim. (Cl. 323-79) The present invention relates to a voltagedivider, and more particularly to an improved voltage. divider having aconstant RC product.

In many applications it would be desirable to have manually andmechanically controlled voltage dividers which have a wide frequencyresponse. Ideally such voltage dividers should have zero frequencydistortion, zero phase shift, infinite resolution, and be capable oflinear or prescribed non-linear operation.

In accordance with the present invention it is proposed to provide avoltage divider, which to a large degree satisfies the foregoingspecifications, by a construction employing a pair of fixed electrodesand. a movable electrode immersed in a liquid dielectric which has aloss characteristic, and hence produces a voltage divider havingaconstant RC product.

It, therefore, is an object of the present invention to provide animproved voltage divider having infinite resolution which is simple andeconomical to manufacture.

A still further object of the invention is to provide an improveddielectric voltage divider which for a given frequency range has zerodistortion, zero phase shift and infinite resolution.

Still another object of the invention is to provide an improveddielectric voltage divider of infinite resolution and constant RCproduct having the desired or prescribed characteristic of operation.

Other and further objects of the invention subsequently will becomeapparent by reference to the following. description taken in conjunctionwith the accompanying drawing, wherein:

Figure 1 is a diagrammatic representation of a voltage divider orpotentiometer constructed in accordance with the principles of thepresent invention;

Figures 2 and 3 are further diagrammatic representations of another formof voltage divider or potentiometer contemplated by the presentinvention;

Figure 4 is a further diagrammatic representation of a type ofdielectric voltage divider contemplated by the present invention;

Figures 5 and 6 are still other diagrammatic representations of afurther embodiment of the present invention;

Figure 7 is a graph representative of the characteristics of the voltagedivider shown in Figure 4;

Figure 8 is a graph representative of the characteristics of the voltagedivider illustrated in Figures 2 and 3; and

Figure 9 is a graph representative of the characteristics of the voltagedivider shown in Figures 5 and 6.

In accordance with the present invention a dielectric voltage dividerhas been provided which has a constant RC product. This is accomplishedby the simultaneous variation of capacitance and resistance with respectto a movable electrode immersed in a liquid dielectric having a constantloss characteristic and located between another pair of electrodesimmersed in the liquid dielec- "ice versely as the configuration orrelation between the elec-.

trodes. is changed. Each elementary capacitive. path between. theelectrodesmust be compensated by an identical resistive leakage path.This. makes it imperative that any solid dielectric used as insulationshould. be. at a minimum and should be placed at locations where theelectric field is weak and substantially constant. In. many applicationsit might be desirable to employ a metal housing for the liquid to reducethe effect of stray capacitive coupling.

An elementary voltage divider is schematically illustrated in Figure 1having a casing 10 which is connected to ground by a conductor 11. Thecasing 10 preferably is a metal housing so as to suitably contain theliquid and minimize the effects of stray capacitance. The container 10is filled with a liquid dielectric having a certain loss characteristic.Immersed in the liquid dielectric are two fixed electrodes A and B,between. which is located a movable electrode D. A suitable source ofpotential E supplied from a source 11 having one terminal connected toground is applied to the electrode. A. The electrode B is connected toground and the casing 10. The movable electrode D is connected to oneoutput. terminal 12, the other output terminal, 13 being connected toground. It now may be assumed that the movable electrode D is thecontrol electrode from which the output is taken, and that it is movablein a vertical direction. Thus there is obtained. a variableresistive-capacitive voltage divider where the ratio: of the outputvoltage to the input voltage is given by the following expression:

ii 1 +jRgwCz) Where:

Z =impedance between D and ground Z =irnpedance between D and A R=parallel resistance between D and ground R =parallel resistance betweenA and D C =parallel capacitance between D and ground C =parallelcapacitance between A and D Note that R C =R C for all positions of' Dat all frequencies.

From the foregoing it is apparent that the output voltage has zero phaseshift and zero frequency distortion, and hence the dielectric voltagedivider is suitable for broadband frequency applications.

The foregoing example and analysis assumes the ideal condition where theresistances R R and the canacitances C C exist solely because of thedielectric fluid. In a practical embodiment, stray capacitances alsoexist between the output terminals and between the other connections tothe voltage divider. Generally these stray capacitances are fixedquantities, and hence the divider can be compensated by the use ofsuitable resistors. In Figure 1 the stray capacitances have beenindicated by the capacitors 14 and 15 having dotted line connections tothe various electrodes A, B and D. A suitable resistor 16 is connectedacross the capacitance 14 and another resistor 17 is connected acrossthe capacitance 15. The output terminals now would be identified asterminals 18 and 19. The resultant arrangement would still provide avoltage divider having a constant RC product. It will be appreciatedthat the addition of the resistors 16 and 17 constitutes a resistiveloading which changes the ratio of output to input voltage as a functionof the control setting. This, however, can be taken into account whendesigning the electrode configuration for a particular controlcharacteristic. which it is desired to achieve.

In certain other applications of a dielectric voltage divider of thiskind it might be impossible to avoid a resistive loading of the output,in which case it may be desirable to deliberately load with capacitorsthe voltage divider in order to provide the desired RC product.

The foregoing analysis has assumed that the dielectric constant of theliquid and resistance characteristic remain fixed under all conditions.It, of course, will be appreciated that some dielectric fluids mighthave characteristics which change as a function of temperature orfrequency. in general it has been felt, however, that where a singlepure dielectric is employed, the characteristics are substantiallyconstant. For the greater part the alcohols have shown the greatestpromise, although acetone and ethyleneglycol also have been employed.

For a voltage divider where the movable electrode is actuatedsubstantially in the manner shown in Figure 1, the output characteristicwould be linear assuming a constant input voltage for the signal source11.

Another embodiment for the invention is schematically illustrated inFigures 2 and 3 showing a casing 21 containing fixed electrodes 22 and23 connected to suitable terminals 24 and 25 cooperating with a groundedterminal'26. The terminals 24, 25, and 2.6 are for connection to acenter-tapped voltage source. A movable electrode 27 arranged to rotatein a circular path perpendicular to the electrodes 22 and 23 isconnected to a terminal 28. The movable member 27 is insulated from theliquid except for its tip end indicated by a circle in the drawing. Thecooperating output terminal 29 is connected to ground. This embodimentproduces a nonlinear relation between the input and output and isgenerally referred to as a sine-cosine potentiometer. The relationshipbetween the constant input voltage and the output voltage expressed interms of angular rotation of the member 27, is shown in Figure 8, fromwhich it will be seen that the curve is a true sinusoidal curve. In oneembodiment the dielectric materials employed were of the alcohol familywhich might be isopropyl, ethyl or methyl. Such voltage divider hadinfinite resolution and operated over a frequency range from zero to 400cycles per second without distortion or phase degradation. The inputresistance was about 50,000 ohms, and the output resistance was1,000,000 ohms.

In another embodiment it is desired to provide an approximatelylogarithmic response function such as that illustrated in the graph ofFigure 7. Therein it is shown that the input voltage was constant andthat the output voltage is shown as the logarithm plotted against a dialsetting or indication of the linear movement of the movable electrode.Such structure is diagrammatically shown in Figure 4 wherein there is acontainer 31 which might be of metal and which is connected to ground.The container has a fixed electrode 32 which is connected to an inputterminal 33, the other input terminal 34 being connected to ground.Mounted on the end of a movable rod 35 is a movable cup-shaped electrode36 which is connected to an output terminal 37 the other output terminal38 being connected to ground.

For convenience in manufacture the fixed electrode 32 is actually asolid element having a circular top surface, while the movable electrode36 is a hollow cylinder with the remote end closed by a plate. Foraccurate control of the linear movement of the movable electrode 36, theshaft 35 is driven through a suitable gearing arrangement, notillustrated, which is coupled to an indicator and dial.

In a still further application of the dielectric voltage divider, it wasdesired to have an output characteristic similar to that shown in thegraph of Figure 9 which shows the output voltage as compared to thepercent of current in one of two actuating coils which rotate themovable electrode. This arrangement is shown in Figures 5 and 6, whereinthere is a container 41 which is connected to ground and to one of theinput terminals 42. The other terminal 43 is connected to an electrode Afixed within the container 41. Another similar electrode B is connectedto ground. One output terminal 44 likewise is connected to ground, andthe other terminal 45 is connected to a sector-shaped fixed electrode D.A movable electrode F is suitably supported on a shaft 46 and actuatedby an electromagnetic device. The movable electrode Fhas two vanes, oneof which is always associated with the fixed electrode D. The other vaneis positioned so as to be closer or farther away from the two fixedelectrodes A and B.

Supporting shaft 46 for the vane electrode F is actuated by a rotor 47having a plurality of permanent magnets arranged in opposite directionsso that one north pole is at the bottom and the other north pole is atthe top. Two actuating coils 48 and 49 are provided which are suppliedwith direct current to position the movable electrode F to produce thedesired output voltage.

One application of the voltage divider shown in Figures 5 and 6 was in acompressor amplifier where the divider was preceded by a cathodefollower and succeeded by a cathode follower. A constant potentialsource of direct current was used as reference and supplied to a directcurrent amplifier. A portion of the alternating current from the lastcathode follower was passed through a rectifier to control the operationof the direct current amplifier which supplied current to the coils 48and 49 to control the position of the movable electrode F. Such anarrangement constitutes a substantially distortionless regulator havingsuificient loop gain to minimize the changes in output level. Thecompression range of such system is approximately 35 db, and the usefulfrequency extends beyond one megacycle.

While for the purpose of illustrating and describing the presentinvention certain preferred embodiments have been shown, it is to beunderstood that the invention is not to be limited thereby since suchother embodiments are contemplated as may be commensurate with thespirit and scope of the invention set forth in the accompanying claim.

I claim:

A voltage divider having capacitance and resistance comprising a casingcontaining a liquid dielectric having a finite resistancecharacteristic, at least two of parallel fiat fixed electrodes immersedin said dielectric liquid, and a third immersed electrode movable in acircular path between said fixed electrodes, said path beingperpendicular to the planes of said fixed electrode.

References Cited in the file of this patent UNITED STATES PATENTS1,755,739 Kositzky Apr. 22, 1930 1,838,987 Cooper Dec. 29, 19312,036,084 Roder Mar. 31, 1936 2,087,003 Miller July 13, 1937 2,147,728Wintringham Feb. 21, 1939 2,176,451 Berndt Oct. 17, 1939 2,419,111Bostwick Apr. 15, 1947 2,659,039 Bourgonnier Nov. 10, 1953 2,720,626Wing Oct. 11, 1955

